Our project focuses on the biology, cell biology, molecular biology and biochemistry of activated lymphocytes, cells that are used for immunotherapy, especially for cancer treatment, and involved in the cellular immune response to xenogeneic tissues. In addition, we have initiated studies examining conditions of expression of porcine endogenous retrovirus under conditions that would be expected to be encountered in clinical xenotransplantation, with particular emphasis on cells of the immune system. In our study on the regulation of human natural killer (NK) and IL-2-activated NK (lymphokine-activated killer or LAK cells), we have shown that signaling through Fas or cell surface adhesion molecules can induce apoptosis of IL-2-activated but not native NK cells to show that the Fas-mediated activation-induced cell death of NK cells is regulated through an oxidation-reduction sensitive pathway involving phosphatase activities. Recently we have further explored the role of redox in regulating the Fas system in NK cells. FasL is a cell surface receptor on many cells, including NK cells, and following engagement of the Fas receptor on the target cells, is one mechanism by which NK cells lyse targets. We have made and explored the observation that FasL expression is also regulated by redox. Our data show that oxidative stress suppresses FasL induction through suppression of a calciuneurin-NFAT-dependent pathway. Moreover, we have found that human NK cells express endothelial nitric oxide (NO) synthase, and low levels of NO. We have further shown that these low levels of NO protect them from activation induced cell death through a mechanism involving activation of the transcription factors, NFAT. This appears to be a mechanism by which some level of lytic activity by activated NK cells can be maintained, despite the occurrence of activated cell death in such activated cells. We have expanded our studies of xenogeneic immune responses in the human anti-pig response in vitro. Our data show that NK cells are the primary mediators of the human vs. pig cytolytic cellular response, and that this response is up-regulated by several human cytokines, including IL-2, IL-12, IL-15, all of which would be expected to be produced in a xenograft recipient. In addition we are continuing to study how the NK-mediated xenogeneic response is modulated by redox conditions. Finally, recent data suggest that certain porcine cells not previously identified as doing so constitutively produce porcine endogenous retrovirus, which can then infect human cells in vitro.